(1) Field of the Invention
The present invention relates to a thin film capacitor for use in an integrated circuit, and more particularly to a thin film capacitor wherein an increase in the leakage current at lower end portions of a lower electrode is suppressed. The present invention also relates to a method for fabricating such thin film capacitor.
(2) Description of the Related Art
A conventional thin film capacitor for a semiconductor integrated circuit of the kind to which the present invention relates is constituted by a structure in which electrodes are formed of polysilicon and a silicon oxide film and a silicon nitride film are stacked as a dielectric layer between the electrodes. A technique for forming a capacitor after the formation of transistors and bit lines in a dynamic random access memory (DRAM) is disclosed, for example, in International Electron Devices Meeting Digest of Technical Papers, 1988, pp 592-595.
In the above conventional thin film capacitor, there is a limit to the extent in which a capacitor area can be sealed down to meet a need for increasing integration density in recent integrated circuits. Therefore, it is forced to effect the reduction of capacitor areas by making films thinner and dielectric constant higher or using a three dimensional structure at dielectric portions of the thin film capacitor. The dielectric films forming the conventional capacitor are silicon oxide films or silicon nitride films whose dielectric constant is at the most 7 so that, in order to achieve the capacitance needed, the films are required to be so thin as 5 nm or less in a value converted to a silicon oxide film. On the other hand, by using such a thin film, it is extremely difficult to realize the dielectric films that have current-voltage characteristics with currents lower than tolerable leakage currents. Even where the three dimensional structure is used in an attempt to reduce an effective electrode area, the lower ends of the dielectric film at sides of the lower electrode inevitably become so thin that the leakage current is caused to increase due to the concentration of electric fields.
Therefore, it may be attempted to realize the required capacitance by using films having larger thicknesses than in the case of, for example, silicon oxide films. To this end, for the formation of the capacitor portion, use may be made of SrTiO.sub.3 having a dielectric constant close to 300 in a room temperature, (Ba,Sr)TiO.sub.3, Pb(Zr,Ti)O.sub.3 or Pb(Mg,Nb)O.sub.3 having a still higher dielectric constant, or a dielectric material of a high dielectric constant typically represented by Pb(Mg,W)O.sub.3. For the lower electrode, use may be made of Pt/Ta, Pt/Ti or RuO.sub.x which suppresses diffusion of silicon and does not form a low dielectric constant film even under an oxygen atmosphere during the deposition of high dielectric films.
However, the above attempt fails because, at the lower end portions of the side faces of the lower electrode, it is unavoidable for the thickness of the dielectric film to be reduced and for the leakage current to be increased due to the concentration of electric fields. This is attributable to the fact that the dielectric material of high dielectric constant and the Pt/Ta of the lower electrode are subjected to physical vapor deposition methods such as, typically, sputtering or evaporation methods and also to the fact that the processing residue having occurred at side portions of the mask during the dry-etching results in rugged surfaces around the processed patterns after the removal of the resists.